1. Field of the Invention
This invention relates generally to series/parallel LED drive systems, and more particularly to techniques designed to protect low voltage current sinks that might be used with such systems.
2. Description of the Related Art
LED lighting strategies may employ LEDs driven in series, parallel, or both. LEDs driven in series by definition all share the same current. If all LEDs share the same current, ideally the brightness of the LEDs will be matched. Some applications require a number of LEDs to be driven with matched brightness, and so connecting the LEDs in series accomplishes the task. A problem can arise, however, if a very large number of LEDs must be driven in series. The series-connected LEDs are powered by a line voltage necessary to provide the necessary current; however, finding line regulators able to support the large line voltage needed for a high LED count series string may be difficult or cost prohibitive.
LEDs may also be arranged in parallel-connected ‘strings’, each of which is driven by a current source or (most commonly) a current sink circuit. But brightness matching between the parallel-connected LEDs is limited by the imperfect matching of the drive circuits, which can vary widely depending on the choice of sink implementation. A parallel LED configuration does have the advantage of typically requiring a lower line voltage than does a series configuration, which may be a benefit in some applications. Also, in some applications LEDs are connected in parallel because different currents need to be driven through the LEDs.
Due to the issues noted above, the best approach may be a compromise between the series and parallel solutions: a “series/parallel” solution. Note that a series/parallel solution could in principle be implemented by simply taking the series approach discussed above and creating multiple copies of this solution. However, this cut and paste approach adds cost to the overall solution because of the need for separate line regulators for each string (or “channel”). In some cases a single integrated circuit (IC) with multiple regulator channels may be able to take the place of multiple regulators, but for a number of solutions an appropriate multiple output regulator may not exist or may still be cost prohibitive due to the number of non-regulator external components required.
A cost effective compromise employing a series/parallel solution is shown in FIG. 1a. Here, each series LED string 10, 12, 14 has its own independent current sink circuit 16, 18, 20, but all series strings share a common line voltage Vline, which is provided by a voltage regulator 22. The voltage on the current sink circuits (VD0, VD1, VD2) is generally set to be equal to the maximum voltage that a string of LEDs might have on its anode connection. This can be arranged by means of a “minimum” circuit 24, which receives the voltages on each of the current sinks and outputs the minimum voltage of the group. An error amplifier 26 receives the minimum voltage and a reference voltage VDdesired at respective inputs, and provides an output Verr to the feedback input of voltage regulator 22 such that the current sink circuit with the minimum VD voltage operates at a desired target voltage equal to VDdesired. This arrangement may result in voltages VD0, VD1 and VD2 being relatively high, necessitating the use of current sinks with a high voltage rating. However, high voltage current sinks are quite costly in terms of silicon die area.
High power zener diodes might be employed to clamp the voltages on each current sink circuit and thereby prevent them from exceeding a safe level. However, this can create high currents through the LEDs which may damage them or reduce their lifetimes.
High voltages on current sink circuits 16, 18 and 20 may also occur if the output of voltage regulator 22 is subjected to a sudden line or load change. In this case, it may take error amplifier 26 a relatively long time to adjust the power delivery. During this time, voltages VD0, VD1 and VD2 may rise to levels that can damage current sink circuits 16, 18 and 20, especially if low cost low voltage current sink circuits are used. This is illustrated in FIG. 1b. A step increase 27 in the Vin supply voltage provided to voltage regulator 22 causes a spike 28 in Vline, which results in corresponding spikes 29 in the VD voltages (VD1 and VD2 shown). Error amplifier output Verr begins to compensate, but not before VD1 and VD2 rise to nearly 12.5V. If current sinks 16, 18, 20 had a maximum voltage rating of 6V, it is likely that significant damage would have been done.